Experts believe it may be the remnants of a vast underwater city, a Japanese Atlantis. We explore this giant pyramid beneath the waves, with drainage ditches, rooms carved in rocks, massive stone steps and even perhaps a once bustling boulevard. But were these strange shapes carved by human hands or the waves? Masaaki Kimura, the leading expert on the underwater monument, calls it the oldest building on earth. "We have been able to collect relics, stone tools...direct evidence that humans existed," he says. There are indeed several similar monuments on land, ancient castles that formed the heart of the Okinawan empire. The region's traditional graves were also often built beside the sea. Perhaps the monument was the victim of rising sea levels as the ice age ended